1. Field of the Invention
The present invention relates to integrated circuit manufacturing, and more particularly to insulated-gate field-effect transistors.
2. Description of Related Art
An insulated-gate field-effect transistor (IGFET), such as a metal-oxide semiconductor field-effect transistor (MOSFET), uses a gate to control an underlying surface channel joining a source and a drain. The channel, source and drain are located in a semiconductor substrate, with the source and drain being doped oppositely to the substrate. The gate is separated from the semiconductor substrate by a thin insulating layer such as a gate oxide. The operation of the IGF BT involves application of an input voltage to the gate, which sets up a transverse electric field in order to modulate the longitudinal conductance of the channel.
Polysilicon (also called polycrystalline silicon, poly-Si or poly) thin films have many important uses in IGFET technology. One of the key innovations is the use of heavily doped polysilicon in place of aluminum as the gate. Since polysilicon has the same high melting point as a silicon substrate, typically a blanket polysilicon layer is deposited prior to source and drain formation, and the polysilicon is anisotropically etched to provide a gate. Thereafter, the gate provides an implant mask during the formation of source and drain regions by ion implantation, and the implanted dopants are driven-in and activated using a high-temperature anneal that would otherwise melt the aluminum.
The threshold voltage (V.sub.T) is the minimum gate voltage required to induce the channel. For enhancement-mode devices, the positive gate voltage of an N-channel device must be larger than some threshold voltage before a conducting channel is induced, and the negative gate voltage of a P-channel device must be more negative than some threshold voltage to induce the required positive charge (mobile holes) in the channel.
Extensive efforts are being directed towards miniaturizing IGFETs in order to increase the speed and density of the integrated circuit chips in which they are used. To increase speed, the saturation drain current (I.sub.DSAT), also referred to as drive current, must often be increased to allow faster charging and discharging of parasitic capacitances. In short channel devices, the drive current is proportional to the difference between the gate-to-source voltage (V.sub.GS) and the threshold voltage. Shifting the threshold voltage towards zero increases the drive current, and shifting the threshold voltage away from zero decreases the drive current.
A threshold adjust implant that adjusts the doping concentration in the channel region is often used for precisely controlling the threshold voltage. The threshold adjust implant usually occurs before forming the gate, although threshold adjust implants through the gate have also been reported. As the doping concentration in the channel region increases, the threshold voltage shifts away from zero (thereby decreasing the drive current), and likewise, as the doping concentration in the channel region decreases, the threshold voltage shifts towards zero (thereby increasing the drive current).
Unfortunately, decreasing the doping concentration in the channel region also increases the source-drain off-state leakage current. Although increasing the channel length decreases the leakage current, it also reduces packing density. In VLSI chips such as microprocessors which contain hundreds of thousands or millions of IGFETs, the leakage currents of individual IGFETs are usually limited to about 1 to 10 nanoamps in order to prevent excessive power consumption. To keep the leakage rents within an acceptable range, yet still provide high packing density, VLSI chips usually employ submicron IGFETs with relatively high channel doping, which constrains drive currents and speed.
Accordingly, a need exists for a fabrication process which allows selected IGFETs to have channel doping that differs from that of other IGFETs so that the fastest devices, with the largest drive currents and leakage currents, can be placed in critical speed paths.